Concrete has hitherto been produced by kneading and mixing cement, aggregate, water, and a chemical admixture. Various admixtures have been mixed from the viewpoint of improving properties of cement, properties during kneading and mixing and placing, and various properties after hardening.
In the production of concrete, the mixing ratio among cement, aggregate, an admixture, water and the like sometimes affects workability, for example, in kneading and mixing during concrete production and in concrete placing, and strength, durability, and appearance of concrete after hardening and thus should be carefully determined by taking into consideration also economical efficiency and the like. Further, the order and timing of introduction of these various mixing materials also sometimes affect workability, strength of concrete after hardening and the like. Accordingly, the order and timing of mixing material introduction should also be carefully determined.
In some cases, concrete is produced in concrete production plants so as to be suitable for mass production, and, in another cases, is produced in an actual concrete construction site. Further, the fluidity of concrete produced in a concrete production plant is improved in a concrete construction site.
In general, in introducing a powdery admixture into concrete, a method is sometimes adopted in which a temporary simple plant is installed. This case, however, is rare, and it is common practice to use a method in which an admixture introduction port is provided near a cement discharge gate, and the admixture is hand-introduced at proper timing depending upon the discharge of cement. In this case, introduction work is controlled by instructing unbagging of the admixture and introduction timing by an operating room using, for example, a buzzer or an indicating lamp.
For example, the following documents describe representative techniques concerning concrete production.
(I) “Dobokugakkai 2002nen Seitei Konkurito Hyojun Shihosho (Concrete Standard Specifications, established in 2002 by JSCE” pp. 292-293 “Production and Construction”
““In general, preferably, an expanding material is introduced simultaneously with the introduction of cement. When the expanding material is introduced together with other materials or solely, it should be previously confirmed by a test that the expanding material can be kneaded and mixed satisfactorily and homogeneously. If the introduction timing is different from the planned one, a proper step such as prolonging of the kneading and mixing time should be taken. When lumps and the like are formed by the deposition of the expanding material onto a mixer, lump debris is mixed in the concrete, leading to a fear of causing partial breaking. Accordingly, upon the formation of lumps of the expanding material, they should be immediately removed.””(II) “Nihonkenchikugakkai Koryudo Konkurito No Zairyo, Chogo, Seizo, Seko Shishin (Guideline for Material, Blending, Production, and Construction of High-Flow Concrete, established by Architechtural Institute of Japan)” p. 94““In particular, regarding measuring capability for powder and admixtures not used in the production of JIS products, especially the capability of measuring and introduction which rely upon human power, sufficient care should be taken so as to avoid undervaluation. Further, in many cases, the kneading/mixing time in the mixer should be twice or three times or more larger than that required of JIS products, that is, 60 to 180 sec. It should be kept in mind that, when the kneading/mixing time is twice or three times, the production capacity of the mixer is lowered to one half to one third.””(III) “JIS A 5308 Ready Mixed Concrete, 8.2.1. Measuring method b.”““Measuring of cement, aggregate, and admixture is carried out in terms of mass. The admixture, however, may be measured in terms of the number of bags if the purchaser approves it. When the amount is below one bag, however, measuring should be carried out in terms of mass.””